1. Field of the Invention
This invention relates to a magnetic recording medium, and more particularly to a non-binder type magnetic recording medium useful as a video recording tape and the like.
2. Description of the Prior Art
As magnetic recording media, there have been in wide use so-called coating-type magnetic recording media which are formed by applying dispersion of powdered magnetic material in an organic binder to a non-magnetic substrate and drying it. The coating type magnetic recording media are not suitable for high density recording since the metal oxides used as the magnetic material have a saturation magnetization which is smaller than that of the ferromagnetic metals. Further, the coating-type magnetic recording medium is disadvantageous in that the manufacturing process is complicated and large scale equipment is required for recovery of solvents and prevention of environmental pollution.
In order to meet strong demand for high density recording, there have been developed so-called non-binder-type magnetic recording media having, as the magnetic recording layer, a ferromagnetic film formed by a vapor deposition process such as vacuum deposition, sputtering, ion plating or the like, or a plating process such as electroplating or electroless plating. The non-binder-type magnetic recording medium is advantageous over the coating-type magnetic recording medium in that the recording layer can be formed of ferromagnetic metal having a saturation magnetization larger than that of the metal oxide used for forming the recording layer of the coating-type recording medium and can be formed without non-magnetic material such as the binder included in the recording layer, and accordingly the recording layer of the non-binder-type magnetic recording medium has a higher magnetic coercive force and is smaller in thickness than the recording layer of the coating-type magnetic recording medium. Further, the non-binder-type magnetic recording medium can be manufactured by relatively simple manufacturing process and is free from environmental pollution due to organic solvents. In particular, the recording layer of the non-binder-type recording medium can be thinner than that of the coating-type recording medium by one order of ten and it has therefore been considered to be the better recording medium for high density magnetic recording.
However, the magnetic film (recording layer) of the non-binder-type magnetic recording medium is apt to corrode and is inferior to the recording layer of the coating-type magnetic recording medium in resistance to weathering and rust prevention properties. A particular is that when the recording layer is rubbed with a magnetic head during recording/reproduction, even slight rust on the surface of the recording layer can cause clogging of the magnetic head, which can result in damage to both the recording medium and the magnetic head. When rusting in the recording layer proceeds further, the recording layer is apt to peel off to cause loss of recorded information.
Further, there is a problem with the durability of the non-binder-type magnetic recording medium since it is inferior to the coating-type magnetic recording medium in still durability as used in a video tape recorder, and therefore there is a great demand for improvement thereof.
There have been proposed various methods of improving the resistance to weathering and durability of the non-binder-type magnetic recording medium. For example, it is proposed in Japanese Unexamined Patent Publication No. 50(1975)-33806 to surface-nitride the non-binder-type magnetic recording medium by ion plating process. In Japanese Unexamined Patent Publication No. 53(1978)-30304, there is disclosed a method in which a silicon nitride film is formed on the surface of the non-binder-type magnetic recording medium by sputtering. In Japanese Unexamined Patent Publication No. 53(1978)-85403 is disclosed a method in which a magnetic film is exposed to discharge in a nitrogen atmosphere to form a non-magnetic surface layer. Further, in Japanese Unexamined Patent Publication No. 54(1979)-143111, there is disclosed a method in which a metal nitride film is formed on a magnetic metal film. However, these methods are all unsatisfactory in that the protective layer formed must be large in thickness in order to obtain sufficient resistance to weathering and sufficient durability. When the protective layer is large in thickness, the electromagnetic conversion properties are significantly deteriorated and the advantage of the non-binder-type magnetic recording medium is lost. A non-binder-type magnetic recording medium having a magnetic film of iron nitride and iron, or of iron nitride disclosed in European Pat. No. 8328 or Japanese Unexamined Patent Publication No. 59(1984)-87809 exhibits superior resistance to weathering, but is insufficient in magnetic properties and durability.